A variety of membrane filtration systems are known and many of these use pressurised systems operating at high transmembrane pressures (TMP) to produce effective filtering and high filtrate flux. These systems are highly effective but are also expensive to produce, operate and maintain. Simpler systems using membrane arrays freely mounted vertically in a tank and using suction applied to the fibre lumens to produce TMP have also been developed, however, these systems have been found in the past to be less effective than the pressurised systems.
Examples of such known systems are illustrated in U.S. Pat. No. 5,192,456 to Ishida et al, U.S. Pat. No. 5,248,424 to Cote et al and WO 97/06880 to Zenon Environmental Inc.
The Ishida et al patent describes an activated sludge treating apparatus where air flow is used to clean the outer surface of the filter membrane. In this arrangement the air blower used for biological treatment of the waste water is also used as a secondary agitation source to clean the surface of the membranes. The membrane modules are of the plate type. The membranes also have a low packing density and thus do not have the problems associated with cleaning tightly packed fibre bundles. Air is bubbled from beneath the modules and is supplied externally from the membrane array.
The Cote et al patent again describes a system of cleaning arrays of fibres. In this case the fibres are mounted in a skein to form an inverted U-shaped or parabolic array and the air is introduced below the array to produce bubbles which contact the fibres with such force they keep the surfaces relatively free of attached microorganisms and deposits of inanimate particles. The fibres are freely swayable as they are only attached at either end and this assists removal of deposits on their outer surface. The bubbles of gas/air flow are provided from a source external of the fibre bundle and move generally transverse to the lengths of fibre. This limits the depth of fibre bundle which can be effectively cleaned.
The invention disclosed in the Zenon Environmental, Inc. PCT Application No. WO 97/06880 is closely related to the Cote et al patent. In this document the fibres are unconfined, vertically arranged and dimensioned to be slightly longer than the distance between the opposed faces of the headers into which the fibre ends are mounted to allow for swaying and independent movement of the individual fibres. The skein is aerated with a gas distribution means which produces a mass of bubbles which serve to scrub the outer surface of the vertically arranged fibres as they rise upwardly through the skein.
Our own International Patent Application WO96/07470 describes an earlier method of cleaning membranes using a gas backwash to dislodge material from the membrane walls by applying a gas pressure to the filtrate side of the membranes and then rapidly decompressing the shell surrounding the feed side of the membranes. Feed is supplied to the shell while this gas backwash is taking place to cause turbulence and frothing around the membrane walls resulting in further dislodgment of accumulated solids.